Boron, bismuth co-doping of gallium arsenide and other compounds for photonic and heterojunction bipolar transistor devices

ABSTRACT

Isoelectronic co-doping of semiconductor compounds and alloys with acceptors and deep donors is used to decrease bandgap, to increase concentration of the dopant constituents in the resulting alloys, and to increase carrier mobilities lifetimes. For example, Group III-V compounds and alloys, such as GaAs and GaP, are isoelectronically co-doped with, for example, B and Bi, to customize solar cells, and other semiconductor devices. Isoelectronically co-doped Group II-VI compounds and alloys are also included.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

The present non-provisional utility application claims priority under 35U.S.C. §119(e) to provisional application No. 61/311,513 titled “Boron,Bismuth co-doping of Gallium Arsenide and Other Compounds for Photonicand Heterojunction Bipolar Transistor Devices,” filed on Mar. 8, 2010,which is hereby incorporated herein by reference in its entirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08G028308 between the United States Department of Energy andthe Alliance for Sustainable Energy, LLC, manager and operator of theNational Renewable Energy Laboratory.

TECHNICAL FIELD

Aspects of the present disclosure relate generally to the formulationand fabrication of doped semiconductor materials and devices.

BACKGROUND

It is possible to formulate and fabricate semiconductor materials bydoping crystal lattice materials such that an amount of an elementbelonging to one column on the periodic table of the elements, (e.g., anelement having one number of conduction or outer shell electrons), isreplaced with an element from a different column or group on theperiodic table, (e.g., an element having a different number ofconduction or outer shell electrons, usually one column or group removedsuch that it has one more or one fewer outer shell electrons). It isalso possible to use various alloys to form the semiconductor material,(i.e., substitutions on lattice sites by elements from the same column(or group) in the periodic table), to obtain particular semiconductorcharacteristics as needed or desired. For example, particular band gaps,crystal lattice constants, mobility, and the like may be obtained.

U.S. Pat. No. 6,815,736 to Mascarenhas (Mascarenhas), builds on theteachings discussed above, describing a system for isoelectronicco-doping of semiconductor compounds and alloys with deep acceptors anddeep donors to decrease bandgap, to increase concentration of the dopantconstituents in the resulting alloys, and to increase carrier mobilityand lifetimes. Group III-V compounds and alloys, such as GaAs and GaP,are isoelectronically co-doped with, N and Bi, to customize solar cells,thermophotovoltaic cells, light emitting diodes, photodetectors, andlasers on GaP, InP, GaAs, Ge, and Si substrates. The GaAs:N:Bi compounddiscussed in Mascarenhas has great promise; however, difficulties havebeen recognized in practical implementations. For example, Nitrogenacting as a deep acceptor, introduces impurity bands in the vicinity ofthe conduction band of GaAs, which degrades the electron transport inthis band. Thus, the mobility of electrons may be lower than optimal. Atlow N and Bi doping levels (typically <1%), the statistical probabilityfor the occurrence of N—Bi pairs is very low. Therefore, the benefitsattributed to charge and size balancing of the N and Bi isoelectronicimpurities using co-doping that were anticipated in Mascarenhas aredifficult to attain in practice.

Similarly to Mascarenhas, but in the context of transistors, U.S. Pat.No. 6,936,871 to Hase (Hase), discusses a heterojunction bipolartransistor (HBT) employing a Group III-V compound semiconductor havingBi added thereto used for a base layer of a GaAs-based or InP-based HBT.For example, Hase describes a GaAs-based HBT formed by various layers,including a GaAsBi:N base. However, Hase does not appear to recognizethat the proposed emitter base junction formed at a boundary between theGaAs emitter and GaAsBi:N base may have undesirable electrical transportcharacteristics. To wit, co-doping of GaAs with Bi and N will have thebeneficial effect of a Bi doping induced valence band offset (and thus adesirable barrier for blocking back injection of holes from the base toemitter), and the counterbalancing of the concomitant Bi doping inducedlattice mismatch, by the N co-doping; however, Hase does not recognizethat the impurity bands in the vicinity of the conduction band, that areinherently associated with N doping, may not be mitigated by the Bico-doping for the reasons discussed above. Hence, electron transport inthe HBT structure of Hase including GaAsBi:N, will not be optimal.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools, and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

Accordingly, aspects of this disclosure involve isoelectronic co-dopingGaAs with Bismuth (Bi) and Boron (B) in a manner that makes it latticematched to GaAs without introducing significant impurity levels at theconduction band. Hence, aspects of the disclosure provide for increasesin the electronic device quality of the alloy enough to make it useful,as an HBT material.

By way of example and not limitation, an exemplary embodiment includes amonolithic, quadruple junction solar cell comprising a first cellcomprising Ge with a bandgap of about 0.67 eV and a second cellcomprising GaAs that is isoelectronically co-doped with an acceptorelement and a deep donor element to have an effective bandgap of about1.05 eV on the first cell. The embodiment also includes a third cellcomprising GaAs with a bandgap of about 1.42 eV on the second cell and afourth cell comprising InGaP with a bandgap of about 1.90 eV on thethird cell.

Another exemplary embodiment, by way of example and not limitation,includes a two-junction tandem solar cell comprising a monolithic firstcell comprising a substrate with a bandgap of about 1.1 eV and a firstcharge-doped p-n junction between a GaP window layer and a particularBSR layer of GaP:B:Bi. The two-junction tandem solar cell also includesa monolithic second cell adjacent to the first cell, the second cellcomprising GaP that is isoelectronically co-doped with a deep acceptorelement and a deep donor element to have an effective bandgap of about135 eV and a second charge-doped p-n junction.

Other embodiments, by way of example and not limitation, include aheterojunction bipolar transistor (HBT) device that includes an emitterlayer, a base layer having at least one Group III-V compound co-dopedwith Bi and B, and a collector layer.

The exemplary aspect of this disclosure includes semiconductor devices,such as HBTs. In addition to the exemplary aspects and embodimentsdescribed above, further aspects and embodiments will become apparent byreference to the drawings and by study of the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than limiting.

FIG. 1 is a diagrammatic cross-sectional view of a monolithic, quadruplejunction, solar cell including a cell of co-doped GaAs:Bi:B;

FIG. 2 is a graphical illustration of a prior art direct bandgap with anacceptor dopant energy level;

FIG. 3 is a graphical illustration of a prior art direct bandgap with adonor dopant energy level;

FIG. 4 is a graphical representation of a bandgap 40 of a semiconductormaterial that is co-doped with Boron and deep donor 48 (Bismuth)impurities;

FIG. 5 is a diagrammatic sectional view of a heterojunction bipolartransistor; and

FIG. 6 is a diagrammatic sectional view of a heterojunction bipolartransistor according to this disclosure.

DETAILED DESCRIPTION

It has been previously demonstrated that the addition of N to GaAs bydoping can reduce the bandgap of semiconductors. For example, the giantconduction band bowing observed in GaAs_(1−x)N_(x) by Weyers et al.,“Red Shift of Photoluminescence and Absorption in Dilute GaAsN AlloyLayers,” Jpn. J. Appl. Physics 31 (1992) p. L853, appeared todemonstrate that the addition of N to GaAs could reduce the bandgap ofGaAs significantly. The subsequent fabrication of theGa_(0.92)In_(0.08)N_(0.03)As_(0.97) alloy by Kondow et al., “ExtremelyLarge N Content (up to 10%) in GaNAs Grown by Gas-source Molecular BeamEpitaxy,” J Cryst. Growth 164 (1996) pp. 175, utilized that concept tofabricate a semiconductor material with the desired 1.0 eV bandgap. Thepresent disclosure and devices described herein are provided in view ofthe following observations: (i) the N in the alloy creates isoelectronictraps, which have defeated all attempts to utilize such giant bowing ofthe conduction band and (ii) the N in the GaAs does not just induce thebowing of the conduction band of GaAs, but the N impurities alsoparticipate directly in the formation of the conduction band via theformation of a deep acceptor N impurity band.

In recent years, dilute bismide alloys, (e.g. GaAs_(1−x)Bi_(x)) haveattracted a lot of interest due to their unusual properties such asgiant band-gap bowing and spin-orbit bowing and their potentialtechnological applications in high-efficiency solar cells,heterojunction bipolar transistors (HBTs), spintronics, andnear-infrared devices. Incorporating Bi results in the formation ofseveral trap levels above the valence band, which is attributed to Bi—Bipair states, and does not degrade electron mobility in GaAsB, as shownby Kini et al., “Effect of Bi alloying on the hole transport in thedilute bismide alloy GaAs_(1−x)Bi_(x),” Phys. Rev. B83, 075307 (2011),which is hereby incorporated herein by reference in its entirety.

The present disclosure makes note of the following discoveries. First,isoelectronic doping of GaAs with Bi creates deep donor states. Second,the effect of such deep donors on the valence band mirrors the effect ofN on the conduction band. It induces a giant bowing of the valence bandof GaAs via the formation of a deep donor Bi impurity band. Third,isoelectronic doping of GaAs with Bi alone leads to a lowering of theband gap without significant degradation of the electron mobility in theconduction band. Fourth, isoelectronic co-doping of GaAs with both Biand B virtually eliminates the poor conduction band electron mobilitiesand hopping-like transport characteristics, which are inherent in GaAsdoped only with N. Fifth, B doping of GaAs has almost no effect on thebandgap or mobilities in GaAs. Boron is practically electronically inertin GaAs. Sixth, the small size of B serves to counterbalance the largesize of Bi when both are co-doped into GaAs.

Aspects of the present disclosure are applicable to the teachings ofU.S. Pat. No. 6,815,736 titled “Isoelectronic Co-Doping” to Mascarenhas,issued on Nov. 9, 2009, which is hereby incorporated herein by referencein its entirety. The present disclosure is also applicable to theteachings of U.S. Pat. No. 6,936,871 titled “Heterojunction BipolarTransistor with a Base Layer that Contains Bismuth” to Hase, issued onAug. 30, 2005, which is hereby incorporated herein by reference in itsentirety. Namely, the present disclosure involves the substitution of Bfor N in the GaAs, and InGaAs layers and the substitution of B for N inthe GaAs and InGaP heterojunction bipolar transistor structures of Hase.Thus, boron is co-doped with bismuth, rather than nitrogen beingco-doped with bismuth, in the various relevant structures disclosed inMascarenhas and Hase. By way of example and not limitation, in place ofnitrogen, boron is co-doped with bismuth to form GaAs:Bi:B andInGaP:Bi:B based heterojunction bipolar transistor structures.

By way of example, and not limitation a high efficiency, monolithic,quadruple junction, solar cell 10, as shown in FIG. 1, may beconstructed according to the present disclosure. An active,light-absorbing cell 12 comprising a dilute alloy ofGa_(1−y)As_(1−x)Bi_(x)B_(y) (sometimes abbreviated as GaAs:Bi:B) with abandgap of about 1.05 eV is positioned between a Ge cell 11 (bandgap of0.67 eV) and a GaAs cell 13 (bandgap of 1.42 eV in the monolithic,quadruple junction, solar cell 10, which also has a InGaP cell 14(bandgap of 1.90 eV) overlaying the GaAs cell 13 and a Ge substrate 15,which is doped to provide a p-n junction 21 as the bottom Ge cell 11.The solar cell 10 also has a bottom contact layer 16 and top grid 17 tofacilitate the electrical connection of the cell 10 within a circuit(not shown). Other features, such as an anti-reflective (A.R.) coating19, window layer 25 to passivate the surface, contact layer 18 tofacilitate ohmic contacts, and back surface reflectors (BSR) 26, 27, 28,29, 30, are shown. The BSR layers 26, 27, 28, 29, 30 are designed to belattice matched to the regions they surround. In various aspects, theBSR layers 26, 27, 28, 29, 30 have higher bandgaps than the surroundingregions.

When solar radiation 34 irradiates the solar cell 10, the higher energy,shorter wavelength portion of the solar spectrum (e.g., wave-lengths ina range of about 652 nm and below) is absorbed and converted to electricenergy substantially in the top (fourth) cell 14 of InGaP. The lowerenergy, longer wavelength solar radiation is transmitted into the next(third) cell 13 of GaAs. The next to highest energy range of solarradiation (e.g., wavelengths of about 873 nm to 652 nm) is then absorbedand converted to electric energy substantially in the GaAs third cell13, which also transmits lower energy solar radiation to the second cell12 composed of GaAs:Bi:B. Solar radiation in the range of about 1180 nmto 873 nm is absorbed and converted to electric energy substantially inthis second cell 12, while the remaining unabsorbed, lower energyradiation is transmitted to the first or bottom cell 11 composed of Ge.The bottom cell 11 absorbs and converts solar radiation in a range ofabout 1850 nm to 1180 nm to electric energy. Therefore, a monolithic,quadruple junction, solar cell 10 constructed as described above canabsorb and convert enough of the solar radiation spectrum to electricenergy to approach an overall cell efficiency of 40% AM1.

The charge doping of each cell 11, 12, 13, 14 to fabricate all the p-njunctions may be accomplished by any suitable means, including addingimpurity or dopant atoms selected from higher or lower groups on theperiodic table of the elements. By way of example and not limitation,the GaAs:Bi:B cell 12 can be p-type doped with acceptor atoms from GroupII (e.g., Zn or Cd) and n-type doped with donor atoms from Group VI(e.g., S, Se, or Te) to form the p-n junction 22 of the GaAs:Bi:B cell12, as well as the GaAs:Bi:B tunnel junction 32 between the second cell12 and the third cell 13.

The cell p-n junction 23 in the third cell 13 of GaAs and the GaAstunnel junction 33 between the third cell 13 and the fourth or top cell14 can also be fabricated by charged doping with Group VI and Group IIatoms, respectively.

Since Ge is a Group IV element, it can be charge doped with acceptoratoms from Group III elements and donors from Group V elements to formthe p-type and n-type semiconductor materials, respectively, for the p-njunction 21 in the first cell 11 and for the tunnel junction 31 betweenthe bottom cell 11 and the second cell 12. The Ge substrate 15 can alsobe p-type doped with Group III acceptor atoms.

This disclosure, however, is not limited to the particular solar cell 10example structure described above. As discussed below, the principles ofthis disclosure can be used with other alloys and compounds and othersemiconductor devices, including solar cells, having n-p and/or p-njunctions.

Reference is now made to FIGS. 2, 3, and 4, wherein FIGS. 2 and 3illustrate dopant energy levels of conventional charged dopant acceptorsand donors in a semiconductor material. In FIG. 2, the conduction bandenergy E_(c) and valence band energy E_(v) are illustrated for asemiconductor material with a direct bandgap, i.e., wherein the minimumE_(c) and the maximum E_(v) both occur where the momentum vector k=0. Inother words, an electron crossing the bandgap 40 has only to change itsenergy, but not its momentum, as opposed to an indirect bandgap materialin which such an electron crossing the bandgap would also have to changeits momentum. A conventional p-type charged dopant is an acceptor typeatom, which is usually of one group to the left of a host element on theperiodic table of the elements, i.e., one fewer electron in the outerenergy shell. Such a conventional, charged acceptor dopant band 42 isillustrated in FIG. 2, which shows that the conventional, chargedacceptor dopant energy level is closer to the valence energy band E_(v)than to the conduction energy band E_(c). A conventional, charged donordopant band 44, as shown in FIG. 3, is closer to the conduction energyband E_(c) than to the valence energy band E_(v).

In contrast, as illustrated in FIG. 4, the isoelectronic dopant level 48created by the Bi in GaAs:B:Bi is a “deep donor”, i.e., a donor with adopant level 48 that is farther away from the conduction energy bandE_(c) and closer to the valence energy band E_(v). B isoelectronicimpurities, on the other hand, typically do not form bound states (ordeep impurity, or trap states) in GaAs. Therefore, GaAs:B:Bi can belattice matched to GaAs.

An isoelectronic deep donor element or dopant, for purposes of thisdisclosure is an isoelectronic dopant that is less electronegative thanthe host lattice element for which it substitutes. An isoelectronic deepdonor element or dopant may behave as a hole trap and have dopant energylevels which are closer to the valence energy band E_(v) of the hostsemiconductor alloy than to the conduction energy band E_(c) of the hostsemiconductor alloy.

The impurity levels introduced by normally used charged acceptors, suchas impurity level 42 illustrated in FIG. 2, are located typically a fewmeV, e.g., about 20 meV, above the valence band edge E_(v). Likewise,impurity levels introduced by normally used charged donors, such asimpurity level 44 illustrated in FIG. 3, are typically a few meV belowthe conductor band edge E_(c). If the depth of these levels introducedby impurities were to be greater than the room temperature Boltzmannenergy kT=26 meV, then most of the dopants would not be ionized at roomtemperature, thus would not behave as acceptors or donors. Impuritylevels induced by charged acceptors or donors that are much deeper than26 meV are referred to as deep levels.

An impurity level 48 induced by a hole trap, such as Bi, is near thevalence band E_(v) rather than being near the conduction band E_(c).Therefore, an isoelectronic dopant, such as Bi, which forms a hole trapand induces an impurity level 48 that is closer to the valence bandE_(v) than to the conduction band E_(c) is referred to as inisoelectronic deep donor.

Also, it is known in the art that In can be added to the cationsublattice or As can be added to the anion sublattice of Group III-Vsemiconductor compounds or alloys to adjust lattice size. Such additionof In and/or As, usually not more than about 5 atomic percent (at. %) ofthe respective anion or cation sublattice, can be used in any of theisoelectronically co-doped Group III-V compounds or alloys describedabove where adjustment of lattice size is needed or desired. Forexample, such additions may be made to accommodate independentoptimization of the isoelectronic co-dopants while maintaining orachieving desired lattice matching constraints. Therefore, the additionof In and/or As to any of the isoelectronically co-doped Group III-Vcompounds or alloys as described above, are considered part of thisdisclosure. It would be unnecessarily cumbersome to list all suchvariations in the description or claims for each embodiment.Consequently, for convenience in describing and claiming thisdisclosure, references herein to GaAs:B:Bi and GaPB:Bi are considered toalso cover such variations as GaAs:B:Bi:In, and references herein toGaP:B:Bi are considered to also cover such variations as GaP:B:Bi:In andGaP:B:Bi:As.

FIG. 5 depicts an HBT semiconductor device 50 according to an aspect ofthe present disclosure, wherein GaAs:Bi:B is used for a base layer 51.The HBT device 50 includes a substrate 52, a sub-collector layer 53composed of n⁺-GaAs, and a collector layer 54 composed of n⁻-GaAs. Thedevice 50 also includes an emitter layer 55 made of n-GaAs and a caplayer 56 made of n⁺-InGaAs. In various aspects, the various layers aresuccessively stacked on the substrate 52. In one aspect, the substrate52 is composed of a single crystal of GaAs.

An emitter electrode 57 is formed on the cap layer 56. In one aspect,portions of the cap layers are removed to form a mesa structure forfunctioning as a base contact.

In other aspects, a base electrode 58 is in direct contact with the baselayer 51. A mesa structure may also be formed to function as a collectorelectrode 59. The collector electrode 59 is formed on the sub-collectorlayer 53.

In various aspects, the emitter electrode 57, the base electrode 58,and/or the collector electrode 59 may be formed of Ti, Pt, Au, orcombinations and alloys thereof. In other aspects, other suitablematerial may be used. In another aspect, the surface of thesemiconductor device that is not in contact with any of the electrodesis covered with an insulating film 60. In one aspect, the insulatingfilm 60 is composed of Si₃N₄. In other aspects, other suitableinsulating components may be used.

FIG. 6 depicts another HBT semiconductor device 61 according to anotheraspect of the present disclosure, The HBT device 61 is similar to theHBT device 50, however, in this aspect, the base layer 51 is composed ofInP:Bi, while the substrate 52 is composed of InP, the emitter layer 55is composed of n-InP, and the collector layer 54 is composed of n⁻-InP.As discussed above, other suitable compounds and alloys may be co-dopedwith Bi and B to produce other HBT semiconductor devices similar to thedevices 50 and 61.

Unless otherwise stated, the word “about,” when used with a bandgapmeans within 0.2 eV, when used with at. % means within 1.0 at. %, whenused with wavelength means within 0.1 μm, Also, when an atomic speciesin the nomenclature of a semiconductor compound or alloy is separated bya colon from other atomic species, e.g., the B and Bi in GaAs:B:Bi, suchatomic species separated by a colon are considered to comprise a verysmall percentage of the alloy or compound, i,e., about 6 at. % or less,and such nomenclature is sometimes used herein for convenience, but notnecessarily for limitation.

Since numerous modifications and combinations of the above method andembodiments will readily occur to persons skilled in the art, it is notdesired to limit the disclosure to the exact construction and processshown and described above. Accordingly, resort may be made to allsuitable modifications and equivalents that fall within the scope of thedisclosure as defined by the claims which follow. The words “comprise,”“comprises,” “comprising,” “include,” “including,” and “includes” whenused in this specification and in the following claims are intended tospecify the presence of stated features, structures, or steps, but theydo not preclude the presence or addition of one or more other features,structures, steps, or groups thereof.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions, and sub combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions, and sub-combinations as are within their truespirit and scope.

1-9. (canceled)
 10. A two junction tandem solar cell, comprising: amonolithic first cell comprising a substrate with a bandgap of about 1.1eV and a first charge-doped p-n junction between a GaP window layer anda BSR layer of GaP:B:Bi; and a monolithic second cell adjacent to thefirst cell, the second cell comprising GaP that is isoelectronicallyco-doped with a Group III acceptor element and a Group V donor elementto have an effective bandgap of about 1.75 eV and a second charge-dopedp-n junction, wherein the Group III acceptor element is B and the GroupV donor element is Bi to form a GaP:Bi:B crystal lattice.
 11. The solarcell of claim 10 further comprising a charge-doped tunnel junctionbetween the first cell and the isoelectronically co-doped GaP secondcell.
 12. The solar cell of claim 10, wherein the BSR layer of GaP:B:Bihas a higher bandgap than the first p-n junction.
 13. The solar cell ofclaim 11 further comprising a first conductive contact under thesubstrate and a second conductive contact located on the second cell.14. (canceled)
 15. The solar cell of claim 10, wherein content of theGaP:B:Bi crystal lattice is about 5 atomic percent B and about 2.2atomic percent Bi.
 16. The solar cell of claim 10, further comprising: athird cell adjacent to the second cell, the third cell comprising GaPthat is isoelectronically co-doped with an acceptor element and a donorelement to have an effective bandgap of about 2.05 eV and the third cellbeing a charge-doped p-n junction.
 17. The solar cell of claim 16,further comprising a charge-doped tunnel junction of isoelectronicallyco-doped GaP between the isoelectronically co-doped GaP second cell andthe isoelectronically co-doped GaP third cell.
 18. The solar cell ofclaim 17, wherein the second p-n junction of the second cell issandwiched between one or more BSR layers, and the third p-n junction ofthe third cell is sandwiched between a window layer and a particular BSRlayer.
 19. The solar cell of claim 17, further comprising a firstconductive contact on the substrate and a second conductive contact onthe third cell.
 20. The solar cell of claim 16, wherein the acceptorelement in the third cell is B and the donor element in the third cellis Bi to form another GaP:B:Bi crystal lattice.
 21. The solar cell ofclaim 20, wherein content of the other GaP:B:Bi crystal lattice of thethird cell is about 7 atomic percent B and about 4.5 atomic percent Bi.22. A heterojunction bipolar transistor (HBT) device comprising: anemitter layer; a base layer comprising at least one Group III-V compoundco-doped with Bi and B; and a collector layer.
 23. The HBT device ofclaim 22, wherein at least one Group III-V compound is GaAs.
 24. The HBTdevice of claim 22, wherein at least one Group III-V compound is InP.25. The HBT device of claim 22 further comprising at least oneelectrode.
 26. A solar cell comprising: GaP that is isoelectronicallyco-doped with a Group III acceptor element and a Group V donor element,to have an effective bandgap of about 1.75 eV, wherein the Group IIIacceptor element is B and the Group V donor element is Bi to form aGaP:Bi:B crystal lattice.